1. Field of the Invention
The present invention relates to a semiconductor light receiving device, such as a phototransistor and the like, and an electronic apparatus incorporating the same. More particularly, the present invention relates to a semiconductor light receiving device used for a photocoupler, and an electronic apparatus incorporating the same.
2. Description of the Related Art
Phototransistors are used as switching elements, such as an optical switch and the like. The phototransistor performs a switching operation which is triggered by a photocurrent generated in a photodiode provided between a base electrode and a collector electrode. Such a photocurrent is generated by carries excited by light incident on a light receiving region.
When a phototransistor is incorporated in, for example, a photocoupler having a light emitting element, a stray capacitance Cf is caused between the phototransistor and a light emitting diode chip (light emitting diode is hereinafter abbreviated as LED) of the photocoupler. The base region of the phototransistor typically serves as a light receiving region. The surface of the base region is uncovered and exposed. Therefore, when a steeply changing pulse voltage is applied between the phototransistor and the LED of the photocoupler, a displacement current (noise current) caused by electromagnetic noise occurs in the base region of the phototransistor via the stray capacitance Cf, which may cause malfunctions of the phototransistor.
In general, the phototransistor comprises an N type collector layer serving as a collector (C), a P type base layer serving as a base (B), and an N type emitter layer serving as an emitter (E). Typically, even when a minute value of base current Ib is input to the P type base layer, a large value of collector current (output current) Ic=hfexc3x97Ib (where hfe represents an amplification factor and Ib represents a base current) is output. Therefore, in order to prevent malfunctions of the phototransistor due to electromagnetic noise or the like, it is most important to suppress a noise current occurring in the P type base layer and its surrounding region.
As described above, when the phototransistor is incorporated in the photocoupler, a sharply changing, pulse-like displacement voltage occurs between the LED and the phototransistor in the photocoupler. In this case, the displacement voltage causes electromagnetic noise via a stray capacitance Cf therebetween, resulting in a noise current in the P type base layer of the phototransistor. In the case of the occurrence of such a noise current in the P type base layer, the N type collector layer of the phototransistor serving as a collector (C) outputs a collector current Ic represented by:
Ic=hfexc3x97Ib=hfexc3x97Cfbxc3x97d(VCM)/dtxe2x80x83xe2x80x83(1)
where hfe is the amplification factor of the transistor, Ib represents a base current, Cfb represents a stray capacitance between the LED of the photocoupler and the P type base layer of the phototransistor, VCM represents a voltage between the LED of the photocoupler and the phototransistor, and t represents a time.
Thus, when a malfunction is caused in the phototransistor, a large value of collector current Ic is output from the phototransistor in accordance with the above-described expression (1), which may have an adverse influence on external circuitry.
In order to address malfunctions due to such electromagnetic noise or the like, phototransistors may be equipped with a common mode rejection (CMR) characteristic which is used as an indicator representing the performance of the phototransistor.
The CMR characteristic represents an ability to absorb noise as follows. With the CMR characteristic, a noise current, which is input to the phototransistor via a stray capacitance Cf, is caused to flow into the N type emitter layer of the transistor, but not the P type base layer region, when a steeply changing voltage (e.g., a noise signal) is applied between the LED and the phototransistor of the photocoupler. Since the N type emitter layer is designed so that a large current flows, a minute noise current caused by a noise signal can be caused to flow out of the phototransistor quickly. The reason the minute noise current is caused to flow into the N type emitter layer through which a large value of current flows, is that the current passing through the N type emitter layer is not substantially affected by the minute noise current.
Accordingly, in order to improve the CMR characteristic when the phototransistor is incorporated in the photocoupler, the light receiving region of the phototransistor needs to be provided with a metal line which is coupled to the N type emitter layer and thus has the same potential as that of the N type emitter layer.
FIG. 9 is a cross-sectional view showing a structure of a photocoupler incorporating a phototransistor in which malfunctions due to electromagnetic noise are suppressed.
A phototransistor 200 shown in FIG. 9 is configured so that a planer N type semiconductor substrate has an N+ type collector layer 22 and an N type collector layer 30 laminated on the N+ type collector layer 22. An N+ type channel stopper layer 130 is embedded at an outer periphery (along a surrounding edge) of the N type collector layer 30, where the N+ type channel stopper layer 130 is exposed from an outer peripheral surface of the N type collector layer 30. A P type base layer 40 having a predetermined thickness is embedded in a middle portion of the N type collector layer 30 surrounded by the N+type channel stopper layer 130, where there is a predetermined space between the P type base layer 40 and the N+type channel stopper layer 130 and the P type base layer 40 is exposed from a surface of the N type collector layer 30. A surface of the P type base layer 40 receives incident light. An N+ type emitter layer 50 having a predetermined thickness is embedded in the vicinity of an edge of the P type base layer 40, where the N+ type emitter layer 50 is exposed from a surface of the P type base layer 40.
An emitter underlying electrode 70 is provided on a middle portion of the N+ type emitter layer 50. An oxide insulating film 60 is provided around the emitter underlying electrode 70, where the oxide insulating film 60 covers surfaces of the N type collector layer 30, the P type base layer 40, the N+ type emitter layer 50, and the N+ type channel stopper layer 130. An emitter electrode 120 is provided on the emitter underlying electrode 70 and the oxide insulating film 60, where the emitter electrode 120 faces the N+ type emitter layer 50.
A metal guard ring line 90 is provided on the oxide insulating film 60 along each inner edge of regions in which the N+ type channel stopper layer 130 is embedded, where the metal guard ring line 90 is electrically connected to the emitter electrode 120. Further, a metal shield line 80 is provided on the oxide insulating film 60 covering the surface of the P type base layer 40, where the metal shield line 80 is electrically connected to the emitter electrode 120 (in FIG. 9, the connections between the emitter electrode 120, and the metal shield line 80 and the metal guard ring line 90 are not shown). A collector electrode 21 is provided on a surface of the N+ type collector layer 22.
The phototransistor 200 is thus provided with an NPN transistor comprising the N+ type collector layer 22 and the N type collector layer 30, the P type base layer 40, and the N+ type emitter layer 50, which serve as collectors (C), a base (B), and an emitter (E), respectively; and a photodiode having a PN junction at the interface between the P type base layer 40 and the N type collector layer 30 where the N+ type collector layer 22 and the N type collector layer 30 serve as cathodes and the P type base layer 40 serves as an anode.
Now it is assumed that the phototransistor 200 is disposed opposing a photocoupler in which a light emitting element 180, such as an LED or the like, is provided on a substrate 170 (a lead frame may be used instead of the substrate). In this case, electric lines of force 190 are generated directing from the emitter electrode 120 of the phototransistor 200, and the metal shield line 80 and the metal guard ring line 90, which are electrically connected to the emitter electrode 120 so that they have the same potential as that of the emitter electrode 120, to the light emitting element 180 of the photocoupler. The electric line of force 190 functions as a shield for protecting the P type base layer 40 from electromagnetic noise caused when a steeply changing pulse voltage is applied between the light emitting element 180 and the phototransistor 200. The electromagnetic noise causes noise currents in the emitter electrode 120, the metal shield line 80 and the metal guard ring line 90. However, the noise current is caused to flow out of the phototransistor 200 quickly.
Thus, the phototransistor 200 of FIG. 9 is provided with the metal shield line 80 having the same potential as that of the emitter electrode 120, which is located on the predetermined region of the P type base layer 40, and the metal guard ring line 90 having the same potential as that of the emitter electrode 120, which is located on the PN junction region between the P type base layer 40 and the N type collector layer 30. Therefore, it is possible to suppress the noise current which otherwise occurs in the P type base layer 40 (the base (B)) of the phototransistor 200 via a stray capacitance Cf between the light emitting element 180 of the photocoupler and the phototransistor 200.
Japanese Laid-Open Publication No. 5-183186 discloses a phototransistor in which a metal guard ring line made of Al is used so as to suppress a noise current due to electromagnetic noise. Japanese Laid-Open Publication No. 11-135824 discloses a photodiode in which metal shield lines 80 is arranged in mesh so as to suppress a noise current due to electromagnetic noise. In the photodiode disclosed in Japanese Laid-Open Publication No. 11-135824, the effect of absorbing electromagnetic noise is improved, however, as the pitch of the mesh is increased, the area occupied by the metal shield line 80 on the light receiving region is increased, resulting in a reduction in a substantive light receiving area and a reduction in the amount of received light. Thus, the output current of the photodiode is not sufficiently obtained.
In recent years, there is a demand for a higher performance and smaller size light receiving device, such as a phototransistor and the like, which requires the increase of the packing density of elements on a substrate. To achieve this, the phototransistor 200 has to secure a certain light receiving area to such an extent that a predetermined amount of light can be received on the light receiving region of the P type base layer 40.
The CMR characteristic of the phototransistor 200 may be improved by providing a number of metal shield lines 80 on the light receiving region on the P type base layer 40. In this case, whereas the CMR characteristic is improved, the increased number of metal shield lines 80 leads to a reduction in the effective light receiving area of the light receiving region of the phototransistor 200. Therefore, a predetermined amount of light cannot be received.
FIG. 10 is an enlarged, cross-sectional view the metal shield line 80 of the phototransistor 200 of FIG. 9 and its vicinity. As shown in FIG. 10, the metal shield line 80 is in the form of a plate having a rectangular section. Therefore, incident light 140 coming to the metal shield line 80 from above is substantially totally reflected as reflected light 150 vertically toward above since the top surface of the metal shield line 80 is horizontal. Thus, a very small amount of light (i.e., including almost only scattering components) enters the P type base layer 40 (light receiving region). As a result, if the number of metal shield lines 80 is increased in order to improve the CMR characteristic, the area shielded from the incident light 140 is increased in the light receiving region of the phototransistor 200. The light receiving area is thus decreased, so that a predetermined amount of received light may not be obtained.
Therefore, it is difficult to simultaneously achieve both an improved CMR characteristic (noise resistance) and a predetermined amount of received light in the phototransistor 200 of FIG. 9.
According to an aspect of the present invention, semiconductor light receiving device is provided, which comprises a semiconductor substrate, a collector region on the semiconductor substrate, a base region embedded in the collector region and exposed from a surface of the collector region, wherein the base region serves as a light receiving region, an emitter region embedded in the base region and exposed from a surface of the base region, an insulating film covering the surface of the collector region, the base region, and the emitter region, a first metal line on the insulating film at a position corresponding to the base region and being electrically connected to the emitter region, and a second metal line on the insulating film at a position corresponding to a junction portion of the base region and the collector region and being electrically connected to the emitter region. The first metal line has a sloped surface such that incident light falling on the first metal line is reflected and directed toward the surface of the base region.
In one embodiment of this invention, the sloped surface of the first metal line is sloped at an angle of more than 45xc2x0 with respect to the surface of the base region.
In one embodiment of this invention, the first metal line and the second metal line are made of Al and Cu.
In one embodiment of this invention, the first metal line and the second metal line are made of Al and Ag.
In one embodiment of this invention, the sloped surface of the first metal line is covered with a protection film.
In one embodiment of this invention, the protection film is made of an oxide film.
In one embodiment of this invention, the protection film is made of a metal film.
In one embodiment of this invention, the metal film is made of Al.
In one embodiment of this invention, the emitter region and the base region each have an electrode portion having a laminate structure comprising an upper layer of Al and a lower layer of Cu.
In one embodiment of this invention, the first metal lines are arranged in mesh.
According to another aspect of the present invention, an electronic apparatus is provided, which comprises the above-described semiconductor light receiving device.
Thus, the invention described herein makes possible the advantages of providing a semiconductor light receiving device having an improved CMR characteristic without a reduction in the amount of received light; and an electronic apparatus incorporating the same.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.